This invention relates to methods and apparatus for monitoring fluid pressure in relatively inaccessible locations, and more particularly relates to improved methods and apparatus for monitoring pressure in a subsea petroleum installation and the like.
It is well known that oil and gas is found in subsurface earth formations, and that wells are drilled into these formations from the surface of the earth, to recover these fluids. Moreover, it is well known that, if petroleum fluids escape from control, they tend to have a seriously damaging effect on the environment.
Any uncontrolled fluid flow from the well is always undesirable, irrespective of where the wellhead may be located, although a leak in a conventional oil well located on land is generally quite easy to detect and correct. Furthermore, enviornmental damage resulting from loss of oil on land is often limited in scope, and much of the oil may be recovered. In the case of a subsea installation, however, escape of the well fluids into the sea is environmentally more deleterious. Furthermore, a subsea discharge is much more difficult to detect, and is alsmost always more difficult and costly to correct. Also, if well fluid escapes into a subsea production control system, the well fluid may contaminate electrical components within such a system and thus damage or destroy the control system. The present invention substantially reduces the probability of such well fluid escaping, yet provides for continuous monitoring of well fluid pressure.
A petroleum installation is an active system having a variety of operating parameters such as flow rates, pressures, etc., which must be continually monitored and controlled in order to achieve maximum recovery of the oil and gas from the subsurface formations of interest. In addition, it is desirable to monitor parameters such as pressure at the wellhead and other locations in the production system as an indication of an abnormality in the petroleum recovery system, so that precautionary measures may be taken. In the case of land installations, wellhead pressure may be measured and observed by conventional gauges and the like which may be conveniently mounted on some suitable portion of the wellhead assembly to provide easily visible readings. In an offshore installation of other remote location, however, the wellhead assembly is positioned on the ocean floor and may be located deep below the surface of the water. Accordingly, the monitoring system will usually comprise a length of conduit which is connected at one end to the wellhead assembly so as to communicate with the fluid to be monitored, and which has its other end connected to a suitable pressure transducer which generates an appropriate electrical signal.
This electrical signal may be delivered to the surface by suitable electrical cables of conventional design, whereby the pressure indications can be observed and recorded, and may also be used to actuate assemblies such as gate valves and the like to control the wellhead pressure. Alternatively, the signal from the pressure transducer may be delivered to a subsea production control module, which contains electrical and hydraulic circuitry to actuate gate valves to control the wellhead pressure.
Subsea electrohydraulic control systems for continuously monitoring and controlling a number of subsea wells may be advantageously located on the sea floor at or near a wellhead. In these systems, control signals from a surface installation may be transmitted to electronic signal processing apparatus located within an enclosure at the sea floor. This apparatus commonly includes several electrically controlled valves that control the flow to hydraulic actuators located at the wellhead assemblies for actuating production flow control valves. In these systems it is desirable to monitor the pressure at various points in the production system at or near the wellhead, which may be accomplished by means of pressure transducers located within a control system enclosure. But it is also desirable that the electronic apparatus be isolated from the production flow lines which may contain corrosive well fluids.
More particularly, the pressure transducer may be mounted within the enclosure or module containing the electronic signal processing apparatus. This module may be suitably mounted on the wellhead assembly in a manner such that the control system module, including the pressure transducer, is readily removable for repair or replacement. Accordingly, the conduit which interconnects the pressure transducer with the production flow lines of the wellhead assembly, and which consequently may be several feet long, would be heretofore filled with the same well fluids which are sought to be recovered at the surface, whereby the pressure transducer is in direct contact with the well fluid and the pressure sought to be measured.
The wellhead assembly and the conduit interconnecting the interior of the wellhead to the pressure transducer are constructed to withstand high pressures, and generally will not rupture under normal design conditions. The problem, however, is that the subsea pressure measuring apparatus is often subjected to abnormal conditions which substantially increase the likelihood of inadvertent leakage. Working in a subsea environment obviously demands special equipment and, partly due to the remote location of the pressure measuring apparatus, it is possible that the conduit interconnecting the pressure transducer with the interior of the wellhead assembly may be damaged by such equipment. Also, installation or repair crews working about the subsea wellhead assembly below the surface may damage the pressure measuring apparatus, and since a well is almost always shut in whenever such work is performed, these crews will usually not realize that damage to the monitoring system has occurred until after they have returned to the surface and the wellhead control valves have been reopened.
The possibility of leakage from the connecting conduit is further increased because of the corrosive nature of the subsea environment. Under some conditions, the connecting conduit is structurally weakened by both chemical corrosion and electrochemical corrosion. Also, because the subsea pressure measuring apparatus is in a remote location, the pressure transducer normally is located within a module containing other electronic signal processing apparatus, and this module may be installed in a more accessible location farther from the interior of the wellhead than in land operations. An increased length of conduit which connects the transducer to the wellhead also increases the likelihood that leakage will occur at some point its length.
Apart from the environmental consequences of a subsea oil leak in the conduit connecting the wellhead to the pressure transducer, such a leak is also very difficult and costly to repair. Underwater repair of inherently much more difficult and expensive than repair operations on land, since qualified repair personnel and extensive equipment must be transported to the remove site. Divers not only require expensive equipment to form underwater operations, but their productivity is reduced because a substantial portion of time is required to become acclimated to the drastic change in ambient pressures. For these reasons it is desirable that subsea repairs be grouped and performed on a scheduled basis and, if possible, unanticipated repair work may be delayed until it can be accomplished at the time of scheduled repair work.
If a major leak develops the conduit connecting the wellhead to the pressure transducer, it may be readily appreciated that well fluid will pass directly to the water resulting in environmental damage and loss of recoverable oil. Further, the environmental consequences of a leak and generally much more serious than the economic loss of oil, and the well has to be shut in for a costly unscheduled repair of the leak to limit the environmental damage.
On the other hand, if there is a minor leak in the conduit connecting the wellhead to the pressure transducer, it may be appreciated that the pressure being monitored by the transducer and forwarded to automatic or manual control apparatus, may not accurately reflect the full pressure at the wellhead. Although oil recovery may be possible under such circumstances it may further be appreciated that the environmental consequences of even a minor leak may necessitate shutting in the well.
As hereinbefore explained, the consequences of a well fluid leak in a subsea oil production installation, and the difficulty and expense of correcting such a leak, is more severe than in the case of a landbased installation. Because of these disadvantages in the subsea pressure monitoring systems presently in use, and especially since governmental agencies have recently adopted regulations which are designed to severely penalize the uncontrolled release of relatively small quantities of well fluid into the environment, there is an increasing need for apparatus that will not only provide effective pressure measurements in a subsea installation, but which will further safeguard against uncontrolled discharges in the event of damage to the monitoring system. Furthermore, this need is especially severe in the case of subsurface petroleum production installations and the like, wherein even relatively minor discharges are a problem because they are not readily detected, and since even a minor leak may produce significant environmental consequences if not detected. Moreover, leaks which occur at even relatively modest depths are much more difficult and expensive to correct, in contrast with similar accidents on land, and this has further intensified the need for a better technique for monitoring wellhead pressures and the like.
As previously explained, it is advantageous that the pressure transducer be located within a module containing other electric signal processing apparatus. It may therefore be appreciated that a leak in the conduit connecting the wellhead to the pressure transducer may also subject the apparatus within the module to the deleterious well fluid. Thus, a leak at the connection between the conduit and the pressure transducer may contaminate the electronic and hydraulic control apparatus within the module with well fluid and damage or destroy that apparatus.
These disadvantages of the prior art are overcome with the present invention, however, and novel methods and apparatus are hereinafter provided for accurately monitoring wellhead and other pressures in subsea petroleum installations and other similarly inaccessible locations.